Fluoropolymer blend anti-reflection coatings and coated articles

ABSTRACT

Solid bodies having a reflective surface are provided with an anti-reflection coating of a fluoropolymer blends of (1) a terpolymer composition derived from (a) perfluoroalkylalkyl acrylate or methacrylate, (b) acrylic, methacrylic or itaconic acid, and (c) hydroxyl-containing acrylate or methacrylate; and (2) an amorphous fluoropolymer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division, of application Ser. No. 796,516, filedNov. 21, 1991 now U.S. Pat. No. 5,118,579 which is acontinuation-in-part of commonly assigned copending U.S. applicationSer. No. 07/762,998 filed Sep. 20, 1991.

FIELD OF THE INVENTION

This invention relates to the field of optical coatings for reducingreflection on reflective surfaces, such as optical surfaces, windows,transparent films, display surfaces, glossy photographs and the like. Itprovides coatings of curable optically clear fluoropolymer blends havinglow refractive index, and articles coated therewith.

BACKGROUND OF THE INVENTION

In any optical element, a portion of the incident light is reflected ateach surface. The exact amount is determined by the refractive indexchange at the dielectric interface. The amount of light so reflected canbe reduced by means of anti-reflection coating. There are four mainreasons why anti-reflection coatings are desirable in optical systems.First, the percentage of reflected light is lowest at normal incidence.The percentage increases with a corresponding increase in the angle ofobservation. This limits the resolution of the image and, in some cases,can completely obscure the image. Second, an increase in reflected lightcorresponds to a decrease in transmitted light. For components such ascompound lenses, this loss is multiplicative and may be intolerable.Third, reflections from optical surfaces often create unwanted ordistracting glare. Finally, for components such as camera lenses andphotographs, that contain many optical surfaces, there are multipleinternal reflections. These reflections can cause stray light to hit theimage plane and thereby reduce the image contrast and definition.

It has been well known for many years that unwanted reflections can besubstantially reduced by providing a surface coating of an opticallyclear coating material having a refractive index which is lower than therefractive index of the substrate. However, difficulty in producing highquality thin films prevented significant practical application untilapproximately 1940, when technology for applying thin films of certainrefractory inorganic materials via evaporation under high vacuumconditions was developed. More recently, low refractive index polymericcoatings, generally fluoropolymer coatings, have been developed foranti-reflection applications. Generally, these coatings, for maximumeffectiveness, are about 1/4 wavelength thick. The basic theory of suchanti-reflection coatings is well known; the technical challenge is inthe provision of conveniently applied, effective, strongly adherent,scratch-resistant and relatively low cost coatings with optimally lowrefractive index, over large areas.

SUMMARY OF THE INVENTION

This invention provides devices comprising a reflective substrate havingdeposited thereon as an anti-reflection coating a thin film of blends of(a) amorphous fluoropolymers with (b) certain cross-linkable terpolymersderived from fluorine-containing acrylic monomers with non-fluorinatedacrylic monomers. These blends can be cured by application of heat toform semi-IPNs (interpenetrating networks) of the cured terpolymer which"trap" the amorphous fluoropolymer component. The cured blends remainamorphous, and they are optically clear. They are based on amorphousfluoropolymers containing only carbon and fluorine, and possiblyhydrogen and/or oxygen, and they have low refractive indexes. Beingsoluble in specific organic solvents, solutions of the uncured blendscan be used to make coatings and to cast films, which arecross-linkable. These coatings are optically clear, robust and stronglyadherent to reflective substrates, including glass, polymer films,metals, crystal substrates and the like.

In accordance with the present invention, there are provided devicescomprising a reflective substrate having deposited thereon as ananti-reflection coating an effective layer of a polymeric compositioncomprising a blend of

(a) from about 1 to about 95 percent by weight of amorphousfluoropolymer, with

(b) from about 5 to about 99 percent by weight of a fluorinatedcopolymer having the general composition ##STR1## wherein R¹ is H,--CH₃, or mixtures thereof;

R² is H, --CH₃, or --CH₂ COOH;

R³ is H, --CH₃, or --CH₂ COOC_(m) H_(2m+1),

wherein m is an integer of from about 1 to about 4;

R⁴ is an alkylene bridging group, straight chain, branched or cyclic,having from 1 to about 8 carbon atoms;

p is 1 or 2;

s, t and u represent weight proportions of the respectivemonomer-derived units, and have values within the ranges of

s=from about 0.5 to about 0.995;

t=from about 0.0025 to about 0.4975; and

u=from about 0.0025 to about 0.4975;

with the sum of s+t+u being 1; and

n is an integer of from about 1 to about 40;

wherein the monomer-derived units may be arranged in any sequence. Inthe above formula, t and u may, but need not be the same.

An "effective layer" of said polymeric composition is a layer ofthickness suitable for reduction of undesirable reflection. This may beevidenced by reduced reflection of the incident light, or by greaterclarity or contrast of the image being observed through said layer, orby an improvement in any other manifestation which may be affected byreduction in reflection of incident light. Generally, though notnecessarily, optimum reflection reduction is achieved when the layer ofsaid polymeric composition is in the order of about 1/4 wavelengththick; other thicknesses, particularly greater thicknesses, are alsoeffective with the fluoropolymer blends employed in the presentinvention.

The term copolymer, as used in the specification and claims, is intendedto refer to a polymer derived from at least two or more, usually derivedfrom at least three different monomer units. There is no theoreticallimit on the number of different monomer units which may be incorporatedinto the fluorinated copolymers for the fluoropolymer blends for theanti-reflection coatings of the devices of the present invention; theirnumber is limited only by the usual practical limitations imposed bypolymerization process considerations, and the desire to obtain polymerproducts having useful properties. Sometimes, these copolymers areherein also referred to as terpolymers.

The copolymer component for the fluoropolymer blends for theanti-reflection coatings for the devices of the present invention mayalso be described as being made up of a polymer chain composed of

    --X.sub.s --Y.sub.t --Z.sub.u ]                            (II)

units wherein s, t and u have the meanings given above in connectionwith formula (I), above, and wherein

X represents monomer-derived units of the composition ##STR2## whereinR¹, p and n, which may be the same or different in individual X unitswithin the polymer chain, have the meanings given in connection withformula (I), above;

Y represents monomer-derived units of the composition ##STR3## whereinR², which may be the same or different in individual Y units within thepolymer chain, has the meaning given in connection with formula (I),above; and

Z represents monomer-derived units of the composition ##STR4## whereinR³ and R⁴, which may be the same or different in individual Z unitswithin the polymer chain, also have the meanings given in connectionwith formula (I), above.

In the copolymers of formula (II), above, the X, Y and Z units may bearranged in any sequence. This freedom of arrangement accordingly alsoprevails for formula (I), above, since formulas (I) and (II) are merelyalternate expressions for the same polymeric compositions.

These copolymers can be prepared by polymerizing the monomers intetrahydrofuran ("THF") or glacial acetic acid at elevated temperaturewith a free-radical generating initiator, using proceduresconventionally employed in making acrylic and methacrylic polymers. Forpurposes of the present invention, they are preferably prepared in apolymerization medium comprising glacial acetic acid or 1,1,2-trichlorotrifluoroethane. Copolymers of that type and their prepration are moreparticularly described in commonly assigned U.S. Pat. No. 5,061,769issued Oct. 29, 1991 to Aharoni.

The term "amorphous fluoropolymer", for purposes of the presentinvention, defines a normally solid polymer having a fluorine-bearingcarbon-to-carbon backbone chain containing carbon, fluorine andhydrogen, and possibly oxygen, and which is amorphous as determined byX-ray diffraction. As conventionally understood, a polymer is amorphousby X-ray diffraction if it shows only the "amorphous halo" and nocrystalline reflections. These amorphous fluoropolymers generally arecopolymers of tetrafluoroethylene (CF₂ ═CF₂) with other fluorine-bearingco-monomers such as, for example, CH₂ ═CHF, CH₂ ═CF₂, CF₂ ═CHF, CH₂═CH--C_(n) F_(2n+1) ; CF₂ ═CF--CF₃ ; CF₂ ═CF--O--C_(n) F_(2n+1) ; CF₂═CF--O--CF₂ CF(CF₃)--O--CF₂ CF₂ COOCH₃, ##STR5##(2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole), and the like. Theco-monomer serves to prevent crystallization. Any amorphousfluoropolymer composition is suitable for use in the blends of thepresent invention, provided it has the requisite solubility in solventsin which the terpolymer compositions are also soluble, and it retainsits amorphicity in these blends and does not phase separate during andafter curing.

Amorphous fluoropolymers are commercially available products. They havea high degree of optical clarity, in combination with the excellentchemical, thermal and electrical properties of conventional crystallineor partially crystalline fluoropolymer, such as polytetrafluoroethylene.They have some degree of solubility in a limited selection of solvents.They are, for example, available from E. I. du Pont de Nemours andCompany under the designation "Teflon-AF" (T. M.). Those provided by duPont, for example, include copolymers of tetrafluoroethylene with2,2-bistrifluoromethyl-4,5,-difluoro-1,3-dioxole (see Polymer Preprints31(1) 312 (1990)).

Coating compositions of these fluoropolymer blends can be prepared byseparately dissolving the amorphous fluoropolymer and theabove-described terpolymer in a suitable solvent for both of thesecomponents, such as, for example, the family of fluorinated solventsprovided by Minnesota Mining and Manufacturing Company (3M) under itsFluorinert® trademark, e.g. perfluoro(2-n-butyl tetrahydrofuran),##STR6## sold under the designation "FC-75" (also described inMacromolecules 10, 1162 (1977)), alone or in combination with1,1,2-trichloro-trifluoro ethane (TCTFE) as a co-solvent. The resultantsolutions can be mixed, in the desired proportions to obtain blendswithin the above-stated composition limits. The resultant mixture canthen be applied to the desired substrate, in the manner described,infra, the solvent evaporated, and the residue polymer blend film can becured (cross-linked) by application of heat to form robust, stronglyadherent coatings.

These polymer blends are optically clear, without haze orinhomogeneities. They have refractive indexes below about 1.4, and aslow as 1.327; good adhesion to glass, silicon, copper foil, polyimide,nylon, polyethylene terephthalate, polytetrafluoroethylene,polychlorotrifluoroethylene and other similar substrates; low surfaceenergy, about half that of polytetrafluoroethylene; excellent thermalstability in air; in combination with good mechanical properties--theyare neither brittle nor elastomeric.

It is an important feature of these fluoropolymer blends that theircopolymer components can be cross-linked by heat treatment without theuse of cross-linking agents. Such heat-induced cross-linking can occureither through internal anhydride formation between two internalcarboxyl groups situated on pendant groups of monomer-derived moieties;or by internal esterification between hydroxyl and carboxyl groups.Heat-induced cross-linking has the advantage that no cross-linking agentis required, so that no impurities are introduced; the cured polymer isa single component with no residual solvent, monomer or cross-linkingagents. The cross-linking process is not associated with creation oflarge voids which can establish optical scattering sites. Suchcross-linking improves hardness, scratch resistance and adhesion of thepolymer blend film, without change in refractive index, and withoutdeleterious effect on any other desirable property. Heat treatmentwithin the temperature range of from about 130° C. to about 150° C. fortime periods of from about 0.25 to about 10 hours, desirably of fromabout 1 to 4 hours, results mainly in esterification; heat treatment athigher temperatures, say within the range of from about 170° C. to about180° C., results in significant anhydride formation. In general,temperatures between about 120° C. and about 180° C. are suitable toeffect cross-linking. As a general proposition, higher temperatures andlonger heat treatment times tend to promote anhydride formation. Heattreatment can be accomplished by placing the coated object in aconventional convection oven, by exposure to infrared lamps, and thelike.

Cross-linking agents may also be employed, if desired, as to bediscussed in further detail below.

The unique properties of these fluoropolymer blends which make them soeminently suitable for use as anti-reflection coatings for opticaldevices are due to the presence in the terpolymer component of thefluorinated moiety in combination with moieties bearing carboxyl groupsand moieties bearing hydroxyl groups. The fluorinated moieties providethe desirable properties of fluoropolymers, and the combination of thecarboxyl groups and the hydroxyl groups provides for processability andcurability, properties which are typically lacking in conventionalfluoropolymers.

Anti-reflection coatings of the above-described fluoropolymer blends areconveniently applied to optical substrates, typically in 1/4 wavelengththickness, by coating the substrate with a solution of the blend,removing excess solution, if any, drying by evaporating the solvent,preferably, but not necessarily, followed by heat-treatment, as abovedescribed, to cure the coating by means of cross-linking. Typicalsubstrates include optical lenses; eyeglasses, both plastic and glass;windows, glass as well as polymeric windows, such as windows of clearpolymeric vinyl (incl. copolymers thereof), styrene, acrylics(Plexiglass) or polycarbonate (Lexan® supplied by General Electric);clear polymer films such as vinyl (incl. copolymers), nylon, polyester,and the like; the exterior viewing surface of liquid crystal displays,cathode ray tubes (e.g. video display tubes for televisions andcomputers); and the like; the surface of glossy displays and pictures,such as glossy prints and photographs; and the like. Determination ofsuitable coating thickness (generally 1/4 wavelength of the light ofwhich reflection is to be minimized) is within the ordinary skill of theart, but is further elucidated, infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood with reference to theannexed drawings, wherein the FIGS. 1-5 provide transmission orreflectance spectra for articles, uncoated and coated in accordance withthe invention, as follows:

FIG. 1 Plexiglass sheet (Example 5);

FIG. 2 plastic polarizer sheet (Example 8) ;

FIG. 3 glossy photograph (Example 9);

FIG. 4 laminated glass (Example 13); and

FIG. 5 polyester film (Example 14); and

FIG. 6 illustrates construction of a transferable anti-reflectioncoating comprising an adhesive-backed optically clear polymer filmhaving an anti-reflection coating.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description sets forth the preferred embodimentsand the best mode presently contemplated for its practice.

Regarding the copolymer components of the fluoropolymer blends, withreference to the "X" units of formula (II), above, which are in moredetail defined by formula (III), above, these are derived fromfluorine-containing acrylate or methacrylate monomers of the formula

    CH.sub.2 ═CR.sup.1 CO--O--(CH.sub.2).sub.p --C.sub.n F.sub.2n+1

wherein R¹, p and n have the meanings given above in connection withformula (I). Those monomers wherein p is 2 are commercially available,as mixtures of homologues having perfluoroalkyl groups of varying chainlength, that is to say as mixtures differing in "n", as they are usuallyobtained in commercial manufacturing operations. Of course, one couldseparate out individual compounds of defined perfluoroalkyl chainlength, if it were desired for any reason. For use in theanti-reflections coating of the present invention, it is preferred touse monomers having a wider distribution of "n", since such widerdistribution makes for better amorphicity, hence greater opticalclarity, as will the use of acrylates (wherein in the above formula R¹is H) with methacrylates (wherein in the above formula R¹ is CH₃). Thosemonomers wherein p is 1 can be readily prepared using known procedures.Preferably, p is 2 and n is an even number. In preferred embodiments, nranges from about 2 to about 30, more preferably from about 4 to about20. Specific examples of preferred embodiments are the products sold byDuPont under its "Zonyl" trademark, e.g. Zonyl TM (the methacrylate) andZonyl TA-N (the acrylate), and sold by Hoechst-Celanese under its"NUVA-HF" trademark. Such specific examples include mixed perfluoroalkylalkyl acrylates and methacrylates wherein n is predominantly an evennumber, and in particular wherein the perfluoroalkyl group isrepresented by a mixture of C₄ through C₂₀ groups, particularly C₆, C₈,C₁₀ and C₁₂ groups.

The "Y" units of formula (II), above, which are in more detail definedby formula (IV), above, are derived from acrylic acid, methacrylic acid,itaconic acid, or mixtures thereof. All of these are commerciallyavailable products.

The "Z" units of formula (II), above, which are in more detail definedby formula (V), above, are derived from acrylic acid esters of theformula

    CH.sub.2 ═CR.sup.3 CO--O--R.sup.4 --OH

wherein R³ and R⁴ have the afore-stated meanings. In more preferredembodiments, R³ is H or --CH₃, with --CH₃ being most preferred. If R³ isrepresented by --CH₂ C_(m) H_(2m+1), then m is preferably an integer offrom about 0 to about 6, more preferably of from about 1 to about 4.With respect to the R⁴ alkylene bridging group, embodiments having from2 to about 4 carbon atoms are preferred, as are the linear and branchedchain embodiments. Use of mixtures of such monomers of differingcarbon-carbon chain length is contemplated. To enhance amorphicity, useof mixtures of such monomers of differing carbon-carbon chain length isdesirable and preferred. Many of the esters suitable for furnishing the"Z" units of formula (II), above, are commercially available; those notso available are readily prepared by those skilled in the art, usingwell-known procedures.

With regard to the weight proportions of the "X", "Y" and "Z" units (seeformula II, above), s ranges from about 0.5 to about 0.995, and t and u,which may be the same or different, each range from about 0.0025 toabout 0.4975. The preferred range for t+u is from about 0.005 to about0.05, with values in the range of from about 0.01 to about 0.03 beingmore preferred yet. As to the weight ratio between t and u (t:u), weightratios in the range from about 1 : 0.5 to about 1 : 1.5 are preferred,with ratios in the range of from about 1 : 0.8 to about 1 : 1.2 beingmore preferred yet. Polymeric compositions of the present inventioncontaining approximately equal proportions by weight of the "Y" and "Z"components have been shown to have desirable properties. If it iscontemplated to subject the polymeric composition to heat-inducedcross-linking, as is preferred to obtain more robust anti-reflectioncoatings, then the Y and Z components are desirably employed in aboutequimolar proportions (rather than in about 1:1 weight ratio). Ifequimolar proportions are employed, then the cross-linking process, asabove described, proceeds predominantly by the internal esterificationroute, with minimal anhydride formation. The esterification route ispreferred because of the better stability of the resultant product inhigh temperature and humid environments.

Polymerization of the monomers to make the polymeric compositions forthe anti-reflection coatings of this invention proceeds readily insolution, desirably in glacial acetic acid or1,1,2-trichloro-trifluoroethane (TCTFE), at elevated temperature withinthe range of from about 35° C. to the boiling point of thepolymerization mixture, more desirably within the range of from about45° C. to the atmospheric pressure boiling point of the solvent, vizabout 47° C. for TCTFE and about 110° C. for glacial acetic acid, underautogenous pressure, typically atmospheric pressure, using a freeradical generating initiator, such as 2,2'-azobis(2-methylpropanenitrile) (CAS #78-67-1) available from DuPont under thedesignation VAZO 64, hereinafter referred to as "AIBN" Other suitableinitiators include 2,2'-azobis(2,4-dimethylpentanenitrile) (CAS#4419-11-8) and 2,2'-azobis(2-methylbutanenitrile) (CAS #13472-08-7).The 2,2'-azobis(2-methylpropanenitrile) is preferred.

The catalyst is employed in amount of from about 0.15 to about 0.4percent by weight, based on the combined weight of all the monomers tobe polymerized. Desirably, polymerization is conducted under drynitrogen atmosphere and with continuous agitation. Typicalpolymerization times range from about 4 hours to about 8 hours. Themonomer concentration in the reaction medium typically ranges from about35 to about 70 percent by weight, based on the combined weight ofreaction medium (glacial acetic acid or TCTFE) and the monomers.

Upon conclusion of the polymerization reaction, the polymer product isreadily recovered from the reaction mixture, as by evaporation of thesolvent and/or cooling the mixture to precipitate the polymer product,followed by separation of liquid and solid phases, as by filtration, andwashing of the polymer product to remove residual unreacted monomersusing any suitable solvent, if desired. These operations areconventional. The polymer thus obtained is soluble inperfluoro(2-n-butyl tetrahydrofuran) of the formula ##STR7## availablefrom 3M as Fluorinert® FC-75, in concentrations of over 10 percent byweight, based on the combined weight of polymer product and solvent.Solution of the polymer product in this solvent is aided by mild heatingand agitation. Mixtures of Fluorinert® FC-75 solvent and TCTFE in up toabout 1:1 vol./vol. ratio are often preferable for rapid dissolution ofthe terpolymer.

The appended claims are intended to cover anti-reflection coated opticaldevices wherein the terpolymer component of the fluoropolymer blendscontain incidental amounts, say up to about 10% by weight of othercomonomers, and particularly of acrylic esters, which do not interferewith the polymerization, and which do not deleteriously affect desirableproperties of the polymer product. Examples of such incidental,additional monomeric materials include alkoxy alkyl acrylates and alkoxyalkyl methacrylates (such as methoxy, ethoxy, propoxy, butoxy and higheracrylates and methacrylates); epoxy alkyl methacrylates; alkyl acrylatesand methacrylates, including haloalkyl derivatives thereof, such aschloroalkyl acrylates and methacrylates; and the like.

When the ratio of the Y-component (acid component) to the Z-component(hydroxyl- bearing acrylic ester) in the polymeric composition of thisinvention is larger than 1.0, then the preferred curing product is theanhydride. When the ratio is smaller than 1.0, an ester is the preferredproduct, with some hydroxyl groups remaining unreacted. When the ratiois 1.0, then the preferred product is the ester, with practically allthe hydroxyl groups being consumed.

Example 1, below, illustrates typical polymerization procedure.

EXAMPLE 1

98.85 g of perfluoroalkylethyl methacrylate monomer mixture (DuPont'sZonyl-TM), 1.0 g hydroxyethyl methacrylate, and 1.0 g methacrylic acidwere polymerized in 148.6 g of glacial acetic acid at 72° C. over a 6hour period, using 0.3 g AIBN as initiator. The polymerizationprogressed remarkably well, and the polymeric product remained solublein the mixture. The polymer precipitated out of the acetic acid attemperatures below about 50° C. The reaction mixture was poured intowater, the polymer precipitated, and recovered. The yield was 80.6%.

The above procedure was repeated, using 1,1,2-trichloro trifluoroethaneas polymerization medium and VAZO®52 as initiator, with comparably goodresults.

EXAMPLE 2

After work-up and drying, the product of the glacial acetic acidpolymerization of Example 1 was dissolved in Fluorinert® FC-75 solventunder heating at about 75° C. to obtain a 5% wt./vol. solution of theterpolymer. Upon cooling, the terpolymer remained in solution; noprecipitation nor haze was noted. Separately, a 5% wt./vol. solution ofamorphous fluoropolymer (du Pont's Teflon®AF-1600 resin) in Fluorinert®FC-75 solvent was prepared at room temperature, and mixed in 1:1vol./vol. ratio with the above-described solution of the terpolymer.This mixture remained absolutely clear. A portion of this mixture wasdeposited on a silicon wafer and dried and heated for 10 min. at 120° C.Thereafter, the refractive index of the coating was found to be 1.3452at 632.8 nm.

EXAMPLE 3

Another portion of the 1:1 vol./vol. mixture of Example 2 was dilutedwith Fluorinert® FC-75 solvent to 0.62% wt /vol. total polymerconcentration and sprayed on glass and on biaxial nylon-6. Followingevaporation of the solvent, the coating was cured by heating to 160° C.for 4 hours. It had a very good visual appearance. The coating was ofsturdy mechanical quality and adhered extremely well to the substrates,in dramatic contradistinction to coatings on these substrates preparedfrom the Teflon® AF-1600 resin alone, which practically fell off thesesubstrates and were very crumbly in appearance.

EXAMPLE 4

A solution of amorphous fluoropolymer (du Pont's Teflon® AF-1600) andthe terpolymer product of Example 1 in 2:1 weight ratio, in a 1:1vol./vol. solvent mixture of Fluorinert® FC-75 solvent and1,1,2-trifluoro trichloroethane was prepared, containing 3 weight % ofthe polymer blend in the mixed solvent. Glass slides, silicon wafers,polyethylene terephthalate (PET) and nylon-6 films were dip-coated withthis polymer blend solution, dried in air, followed by curing at 140° C.for about 1 hour. Solid, adhering coatings were obtained. The refractiveindex of these coatings at 632.8 nm was 1.3275 (average of 2measurements of 1.3273 and 1.3276). Here, again, comparative coatingsprepared from the amorphous fluoropolymer alone fell off the substratesupon drying or just by blowing air on them.

As previously indicated, the polymeric compositions of this inventioncan also be cross-linked employing conventional cross-linking agents,such as, for example, diisocyanates, carbodiimides, diacid chlorides,and the like. Examples of specific effective crosslinking agents includehexamethylenediisocyanate, methylene di-p-phenyldiisocyanate,1,3-dicyclohexyl carbodiimide, dodecanedioyl dichloride and adipoylchloride. The crosslinking agents are employed in amounts conventionallyemployed to obtain desired cross-linking of the polymer which, by use ofsuch agents, can take place at ambient temperatures.

To be effective in suppressing the undesired reflection, theanti-reflection coating should have a refractive index less than that ofthe substrate, or the underlying layer on which the coating is applied,and have the appropriate optical thickness. The optical thickness isdefined as the physical coating thickness times the material'srefractive index. According to the conventional theory of reflection fordielectric interfaces, the reflectivity for normally incident light isgiven by ##EQU1## Therefore, in order to achieve zero reflectivity(numerator set to 0), the ideal coating refractive index is equal to thesquare root of the refractive index of the substrate times the squareroot of the refractive index of surrounding medium. In mostapplications, this surrounding medium is air, which has a refractiveindex of 1. Hence, if the refractive index of the coating material isexactly equal to the square root of the substrate refractive index, allsurface reflection will be eliminated at the wavelength corresponding to4 times the optical thickness. At other wavelengths, while thedestructive interference from the reflected light from the top andbottom coating interfaces will not be complete, substantial reduction inreflection will still be obtained. For most applications, the optimalanti-reflection coating can be obtained by making the optical thicknessone quarter of the mid-point of the visible wavelength range (onequarter of 5500 Angstroms or about 1400 Angstroms). It should be noted,however, that in certain circumstances, it may be desirable to reducethe reflection in a certain portion of the spectrum other than themid-point. This can easily be done by slightly altering the processparameters.

In general, the substrates being coated with the above-describedfluoropolymer blends have a refractive index of at least 1.49. Ideally,the refractive index of the coating material should fulfill the squareroot requirement mentioned above. For example, to optimally coatstandard window glass, which has a refractive index of about 1.5, thecoating material should have a refractive index of about 1.23. To coatmany polyesters, which have a refractive index of about 1.66, thecoating material should have a refractive index of about 1.29. Whilethese polymer blends do not achieve this ideal, their refractive indexis sufficiently low to make them useful for anti-reflection coatingapplications. As an example of this, uncoated substrates with arefractive index of 1.5 have a reflectance of about 4% per surface atnormal incidence. Preferably, reflections below 2% are desired, whichcorresponds to a coating refractive index of less than 1.41; morepreferably, reflections below 1.5% are desired, which corresponds to acoating refractive index of less than 1.38. Most preferred arereflections below 1%, which require a coating refractive index of lessthan 1.35.

Although single layer coatings are sufficient for many applications,they do have limitations. For example, the minimum reflectance, persurface, obtained by using a single polymer blend layer on crown glassis about 0.9%. In many circumstances, this may be unacceptably high. Itis possible to even further reduce, and often completely eliminate, theminimum surface reflection by using multi-layer anti-reflectioncoatings, specifically two layer coatings. One of the trade-offs,however, is that the reflection rises rather sharply away from thewavelength of minimum reflectance. The creation of two layer coatingsinvolves the application of a high refractive index layer onto thesubstrate surface, then the subsequent application of a low refractiveindex layer on top. The refractive index of the high index layer must begreater than that of the substrate, while the refractive index of thelow index layer must be lower than the substrate. It should be notedthat solvent selection is extremely important, so that they arecompatible with the substrate and do not redissolve the other coatinglayers.

Those skilled in the art will realize that unlike with single layercoatings, the thicknesses of each layer in a two layer coating can bemodified over a relatively wide range to produce the desiredanti-reflection coating. The optical thickness of high index layer ispreferably about one quarter to one half wavelength, while the opticalthickness of the low index layer is preferably about one quarterwavelength. In the most conventional two layer coatings, each layer hasan optical thickness of one quarter wavelength. Ideally, in this case,the coating materials are chosen such that the square of the refractiveindex of the high index material divided by the square of the refractiveindex of the low index material is equal to the refractive index of thesubstrate. If this is not possible, it is preferable that the indexdifference between each layer and the substrate be at least 0.1.Quantitative determination of the reflection properties of multilayerdielectric coatings are well understood and known to those skilled inthe art.

Every application has its own glare reduction requirements. Since it isnot possible to attain zero reflectance across the entire spectrum, eachapplication must be analyzed to choose the optimal coating. The goal ofminimizing the total reflectance can be achieved by numericallycalculating the integral ##EQU2## where TR is the total reflectance,R(λ) is the spectral reflectance, I(λ) is the spectral intensitydistribution, S(λ) is the spectral sensitivity of the detector, and λ isthe wavelength. Ideally, the reflectance minimization is accomplished byreducing the reflectance in the wavelength region where I(λ)S(λ) islarge.

The refractive index of the anti-reflection coatings can be determinedas follows: A 15-20 wt % solids solution of the polymer blend in anappropriate solvent is spin cast onto a clean silicon wafer at rotationspeeds between 1500 and 3000 rpm. In general, this will yield a filmthickness between 1 and 3 microns. The fluoropolymer blend coating isthen cured in a convection oven at 150° C.±20° C. for 4 hours. However,it was found that the refractive indexes were unaffected with cure timesas short as 10 minutes and as long as 24 hours. The room temperaturerefractive indexes of the polymer blends are then measured at 632.8 nmusing a Metricon® PC-2000 Prism Coupler.

The substrates used for anti-reflection coated devices include, but arenot limited to, two major categories: inorganic oxides and plastics.Typical inorganic oxides include, but are not limited to, fused quartz,glass (all grades of optical glass as well as any and all commonvarieties), and sapphire.

Typical substrates include optical lenses; eyeglasses, both plastic andglass; windows, glass as well as polymeric windows, such as windows ofclear polymeric vinyl (incl. copolymers thereof), styrene, acrylics(such as Plexiglass) or polycarbonate (Lexan® supplied by GeneralElectric); clear polymer films such as vinyl (incl. copolymers), nylon,polyester, derivatized cellulose, and the like; the exterior viewingsurface of optical (electro-optical) flat panel displays, such as liquidcrystal displays of all types, ac plasma displays, dc gas dischargedisplays, electroluminescent displays, light emitting diodes, vacuumfluorescent displays, and the like; cathode ray tubes (e.g. videodisplay tubes for televisions and computers) and the like; the surfaceof glossy displays and pictures, such as glossy prints and photographs,and the like; and optical indicator components, such as dials, knobs,buttons, windows and the like in environments where reflections are aproblem such as aircraft interiors, aircraft cockpits, automotiveinteriors and the like.

The following describes typical procedures for applying theanti-reflection coatings to make the devices of the present invention.This description is illustrative only, and subject to modification fromcase to case to optimize coating quality and/or to accommodate differentmaterials, as is within the skill of the art.

Prior to the coating operation, the substrates are scrubbed clean withmethanol in an ultrasonic cleaner for at least 30 seconds. Upon removal,they are sprayed with fresh methanol to insure that no contaminationremains on the surface. After being blown dry with filtered nitrogenair, the substrates are baked in an convection oven for about 5 minutesat about 100° C. to remove any residual moisture. No additional surfacetreatment steps are necessary before applying the coatings.

The substrates may be coated either by spin coating or dip coating fromsolutions of the polymer blends described above. Flexible substrates,such as nylon or polyester (Mylar) films are preferably dip coated.Rigid substrates may be coated using both methods. The polymer blendsolution concentrations needed for these applications varied dependingupon the specific polymer blend, the molecular weight of its individualcomponents, and the solvent used. In general, workable polymer blendconcentrations are in the range of 0.5 wt.% to 3 wt.% solids for spincoating and 3 wt.% to 8 wt.% solids for dip coating. It should be notedthat for dip coating, polymer blend concentration variations of 0.1%were found to alter the thickness of the film on the order of 100Angstroms.

The dipping can be performed using a Newport Corporation Actuator 850motorized micrometer attached to a translation base, typically employinga stage range of 1 inch. The actuator may be controlled by a NewportProgrammable Controller 855C. Substrates are dipped into and pulled outof polymer blend solution at rates between 100 and 400 microns/second,where faster pulling rates correspond to thicker films. As anapproximate rule, the thickness of the pulled film increases linearlywith the pulling rate. In our operation, the polymer blend solution wascontained in a vial that was, at most, half full. This allowed for theupper half of the vial to have a semi-solvent atmosphere, giving thefilm time to dry slowly. Dip coating needed to be done in an area withno drafts, since the air currents tended to create streaks on thesubstrates by causing substrate motion and inhomogeneous drying. Unlessspecial precautions are taken, dip coating yields polymer films on bothsides of the substrate.

The spin coating may be performed using a Headway Research photoresistspinner. In our operation, the spinner was enclosed in a Plexiglass boxwith a laminar flow hood mounted on top. Filtered nitrogen air was usedto purge the spinning chamber and keep it reasonably dust-free. Sampleswere spun cast at rotation speeds between 1500 and 3000 rpm.

All initial samples on non-polymeric substrates were cured in aconvection oven at 150° C. for 4 hours. It was found, as with therefractive index measurements, that curing times ranging from 10 minutesto 24 hours did not affect the overall optical properties of the films.Subsequent samples were therefore cured at temperatures between 100° C.and 150° C. for up to 1 hour. For two-layer films, the initial layer wascured for at least half an hour before the second layer was coated. Itwill be recognized that other curing means can be employed, includinginfrared lamps, hot bars, microwave radiation, infrared lasers, as wellas other sources of thermal stimulation.

The thicknesses of the anti-reflection films were measured using twodifferent methods. For glass substrates, a Sloan Dektak IIA profilometerwas used not only for the thickness measurements, but also for anevaluation of the surface roughness. A measure of the thickness couldalso be inferred from the transmission in the spectral data of theanti-reflection coated sample. Since there was a high degree ofcorrelation between the theoretical model and the experimental data, itwas possible to obtain a highly accurate thickness measurement bymatching the experimental wavelength of minimum reflection with thetheory. This latter method was used exclusively with the plasticsubstrates.

All transmission and reflection measurements of anti-reflection coatedsamples were done using a Perkin-Elmer Model 330 spectrophotometer. Thenormally incident transmission measurements were done relative to air.Many of the plastic substrates contained dyes, microcrystallites, orsurface machine grooves that were not removed after manufacturing. Itwas therefore often difficult to infer absolute reflectivity from thetransmission spectra. Reflectance spectra were taken relative to afreshly aluminized quartz slide and calibrated by measuring thereflectance from a clean quartz slide. Due to the geometry of theapparatus, the probe beam had a 6. angle of incidence on the sample.

Glass (of any type, incl. optical glasses as well ordinary windowglass), quartz, and oxide crystals, such as sapphire, are rigidsubstrates that are impervious to all organic solvents. They are,therefore, the most easily processable. Anti-reflection coated samplesare conveniently prepared via either spin coating or dip coating,followed by curing at elevated temperature. Since sapphire has a veryhigh refractive index, the amount of reflected light can besignificantly reduced by simply using a single layer anti-reflectioncoating with materials having a refractive index of approximately 1.34.

Unlike sapphire, both quartz and microscope glass have relatively lowrefractive indexes. While the reflection can be significantly reduced byapplying a single polymer blend layer of the appropriate thickness, itmay be advantageous to use a two layer coating. As stated earlier, thebottom layer consists of a high refractive index material and the toplayer consists of a low refractive index material. The words "highrefractive index" and "low refractive index" are referenced relative tothe refractive index of the substrate.

In general, each layer in a two layer anti-reflection coating can have arather wide range of thicknesses. In the most common embodiment,however, each layer has an optical thickness of one quarter wavelength.The high index layer can be made from a wide range of materials, such aspoly (9-vinyl carbazole) which has a refractive index of approximately1.67. This polymer readily dissolves in many common solvents, such ascyclohexanone.

As an example of the advantages of two layer coatings, consider asubstrate with a refractive index of 1.5. The bare substrate would havea reflectance of 4% per surface at normal incidence. If a single layercoating, with a refractive index of 1.345, were applied at a quarterwavelength optical thickness, the reflectance would decrease to 0.9% persurface at 5500 Angstroms. If the substrate had a two layer coating, lowindex layer with a refractive index of 1.38 and high index layer with arefractive index of 1.67, both with an optical thickness of one quarterwavelength, the reflectance would decrease to 0.02% per surface at 5500Angstroms.

The anti-reflection coating process described above can also be appliedto polymeric materials. In particular, materials that do not dissolve orswell in the solvent systems used for the coating materials, such ascured epoxies, cured polyurethanes, nylon, polyester, Lexan®(polycarbonate), Plexiglass (thermoplastic acrylics), and the like canbe readily coated. Other polymeric materials, such aspolytetrafluoroethylene, may be dissolved or swelled by the solvents orsolvent systems described above. In these cases, an alternate coatingmechanism will need to be employed, such as the use of a differentsolvent system, melt processing, or water based emulsion. In general,when the solubility parameter δ of the solvent or solvent mixture of thecoating is different from (either larger or smaller) the measured orcalculated solubility parameter of the substrate by more than about 1(MPa) ^(1/2), then the solvent mixture will not dissolve the substrate.For example, the δ_(solvent) (MPa)^(1/2) for

TCTFE 14.8

Fluorinert FC-75=12.7

1:1 FC-75/TCTFE=13.8

tetrahydrofuran (THF)=18.6,

1:1 THF/hexafluoroxylene (HFX)=17.2.

Accordingly, the following solubilities will be observed:

    ______________________________________                                        Solvent   Substrate   δ.sub.substrate                                                                   Result                                        ______________________________________                                        TCTFE     PET         21.9      no effect                                     FC-75     nylon 66    27.8      no effect                                     THF       PTFE        12.7      no effect                                     THF       Lexan       20.5 ± 1                                                                             soluble                                       THF       PVC         19.3      soluble                                       MEK*      Lexan       20.5 ± 1                                                                             soluble                                       THF       polystyrene 17.6      soluble                                       THF       poly(methyl 18.6      highly soluble                                          methacrylate)                                                       1:1 TCTFE/                                                                              Lexan       20.5 ± 1                                                                             insol. (no effect)                            FC-75                                                                         1:1 TCTFE/                                                                              PVC         19.3      insol. (no effect)                            FC-75                                                                         TCTFE     Lexan       20.5 ± 1                                                                             insol. (no effect)                            ______________________________________                                         *methyl ethyl ketone                                                     

Incidentally, the solubility parameter δ for a fluoropolymer blend, asabove described, employing a terpolymer of the composition 98:1:1perfluoroalkyl ethylmethacrylate/methyl methacrylate/hydroxyethylmethacrylate (Example 1, above) together with DuPont's Teflon® AF-1600as the amorphous fluoropolymer, in terpolymer:fluoropolymer weight ratioof 1:1, is 13.2 (MPa)^(1/2), so that that fluoropolymer blend is solublein both FC-75 as well as 1:1 vol./vol. FC-75/TCTFE.

EXAMPLE 5

A single layer anti-reflection coating of the composition 1.1 amorphousfluoropolymer (DuPont's Teflon® AF-1600) and the terpolymer product ofExample 1 was applied to a 5 mm thick, 50 mm diameter Plexiglass disk inthe above-described manner via spin coating from a 2.2 wt.% polymerblend solution in Fluorinert® FC-75 onto one side using spin rotation of2000 rpm, followed by curing at 100° C. for 30 minutes. The coatingprocess was then repeated on the other side. Transmission spectra forboth the coated and the uncoated disk are shown in FIG. 1. Significantenhancement of optical transmission is achieved by the coating.

EXAMPLE 6

Most common eyeglass lenses are made out of glass (n=1.51) or a hardcrosslinked polymer resin (n=1.51). In both cases, the materials do notdissolve in the solvents used for the above-described polymercompositions. Single layer anti-reflection coatings were applied to bothtypes of lenses via dip coating, using ˜4 wt.% solution of 1:1 amorphousfluoropolymer (DuPont's Teflon® AF-1600) and terpolymer product ofExample 1 in Fluorinert® FC-75. Transmission spectra were taken for bothuncoated and coated low positive diopter lenses. We observed about 97.5%transmission for a single layer coated lens, while an uncoated lensexhibited about 91.5% transmission. There were negligible differencesbetween the glass and resin lenses.

EXAMPLE 7

There is a move toward higher refractive index materials for eyeglasslenses, to allow for thinner, therefore lighter, lenses with reducedcurvature. Chemically hardened polycarbonate is a material of choice. Asingle layer anti-reflection coating using the polymer blend solution ofExample 6 was applied to a non-prescription chemically hardenedpolycarbonate lens via dip-coating. The lens material had a refractiveindex of about 1.6. Transmission spectra were taken both before andafter the coating operation. We observed about 98.5% tansmission for thesingle layer coated lens, vs. about 89% transmission for the uncoatedlens.

EXAMPLE 8

A single layer anti-reflection coating of a blend of the composition wasapplied using the polymer blend solution of Example 6 by theabove-described dip coating procedure to each of the three components ofthe liquid crystal display window of an electronic calculator (CanonHS-101H) viz. (1) the liquid crystal display; (2) the plastic sheetpolarizer; and (3) the polycarbonate faceplate covering the display andpolarizer. Each of the three components exhibited a significant increasein transmission. The coated faceplate showed 98% transmission at awavelength of 600 nm vs. 89% transmission for the uncoated faceplate atthe same wavelength. The sheet polarizer was designed to selectivelyabsorb one polarization, while being almost transparent to theorthogonal polarization. Therefore, for unpolarized light, we shouldhave observed only 50% transmission less the appropriate transmissionloss. The top curve in FIG. 2 demonstrates this phenomenon. It alsoshows a 4% increase in transmission (reduction in reflectance) afterapplication of a single layer of the anti-reflection coating. Thereflectance of the outer glass surface of the liquid crystal displayalso decreased from about 4.25% for the uncoated component to about 1%for the coated display. When reassembled, the calculator viewing windowexhibited about 13% less reflectance. This was visually demonstrated bya dramatic increase in the contrast of the display.

EXAMPLE 9

When a glossy photograph is viewed under normal lighting conditions, asmall, though significant, portion of the light is reflected back,making the color and the image appear "washed out". To minimize thisproblem, it is usual practice to use a mat surface finish. This,however, reduces the resolution of the image. We have demonstrated thatan anti-reflection coating of a thin layer of the above-describedpolymer blend when applied to a glossy photograph significantly reducesspecular reflection. A glossy black and white photograph containing auniformly black image was partially coated with above-describedfluoropolymer blend using the polymer blend solution of Example 6 viadip coating. The total reflectance, both specular and diffuse, wasmeasured using a Perkin Elmer 330 spectrophotometer equipped with anintegrating sphere. Measurements were performed on both the uncoated andcoated sections of the photograph at a 7. angle of incidence. The data,shown in FIG. 3, demonstrates the substantial decrease in reflectance.

A further effective application of the anti-reflection coated opticaldevices of the present invention involves their use as transparentcovers for read-out instruments and instrument panels, such asautomotive instrument panels. Such panels are commonly tilted, orcurved, to reduce back reflected light. Tilting or curvature tend toreduce overall visibility, and to increase the size of the component.Application of an anti-reflection coating of the above-describedfluoropolymer blend composition effectively reduces reflection andincreases instrument visibility.

Besides spin coating and dip coating, as above described, theanti-reflection coating may also be applied by spray coating and rollercoating. In the former, a fine mist of polymer solution is sprayed ontothe substrate in a semi-solvent atmosphere and allowed to dry slowly.There are numerous parameters that need to be controlled in order to usethis technique: solution viscosity, mist particle size, substratemovement speed, spray area overlap, and sample orientation. In thelatter, roller coating, a squeegee (similar to that used in screenprinting) is used to apply a uniform thin polymer layer.

There are some difficulties in applying a thin polymer layer of uniform,predetermined thickness to large curved objects. A transferableanti-reflection coating could provide a solution. In accordance with thepresent invention, this is accomplished by the provision of a structure(herein referred to as "applique") comprising an optically clear plasticfilm, such as polyester (Mylar) which is anti-reflection coated on oneside, with an refractive index matching adhesive on the other side, asis illustrated in FIG. 6. As shown in FIG. 6, an optically clear plasticfilm 1 has an anti-relection coating 2 applied to one side, and anadhesive coating 3 on the opposite side. The adhesive coating 3 isbacked with peel-away release film 4, which may be conventionalrelease-coated paper. For application, the release film is peeled off,and the anti-reflection coated highly transmissive optical film isapplied to the viewing surface of flat surface displays (as enumerated,supra. , CRT's, VDT's, eyeglasses, etc. This alleviates the problem oftrying to coat cumbersome objects. If damaged, the applique can easilybe removed and replaced. Preparation of such an applique, is illustratedby Example 10, below:

EXAMPLE 10

Cleer-Adheer® sheet (C-Line Products, Des Plaines, Ill.) was used as thebase for an applique. The sheet consists of a mylar film backed with apressure-sensitive acrylate adhesive and peel away release paper. Aportion of the laminating sheet was dip coated with the the polymerblend solution of Example 6 and baked at 100° C. for 60 minutes.Visually, there was a significant difference in light transmission andoptical clarity between the uncoated and coated sections.

EXAMPLE 11

A 0.05 mm thick clear security Llumar Film (All Purpose Glass CoatingCo., Clifton, N.J.), consisting of a polyester film backed with apressure sensitive adhesive and a peel-away plastic backing film, wasused as the base for an applique, A 20×25 cm sheet of this film was laidflat on one side of a wet glass sheet. The water permitted easy movementof the film until it was placed in the proper position. Using a softcloth, the water was pressed out, leaving an adhesive/glass interface.The same process was repeated on the other side of the glass. The glasssheet so treated on both sides was allowed to dry at room temperaturefor 24 hours, after which time it was baked at 60° C. for 4 hours. Theresultant coated glass sheet was clear and free from air bubbles. Thesurfaces were cleaned with soap and water, followed by drying at 100° C.for 10 minutes. The entire glass sheet was then dip coated with asolution of the polymer blend solution of Example 6 and baked at 100° C.for 30 minutes. There was a significant enhancement of opticaltransmission, as measured using a Perkin Elmer 330 spectrophotometer.The average total reflectance near 550 nm for the coated, laminatedglass was approximately 4%, as compared to about 8.5% for the bareglass. The clarity enhancement was not quite as great as that obtainedon a bare glass sheet coated with the polymer blend, possibly due toslight index mismatches between layers.

EXAMPLE 12

A film as described in Example 10, above, is dip coated with thefluoropolymer blend solution employed in Example 6. It is permitted todry and is then baked at 125° C. for 1 hour to cross-link the polymerblend. Thereafter, the film is permitted to cool to room temperature,the backing film is peeled away, and the film without the backing sheetis placed on a wet glass sheet and properly positioned. The water isthen squeezed out using a soft cloth, and the structure is permitted todry at room temperature for 24 hours, followed by baking at 60° C. for 4hours. The laminated glass sheet thus obtained exhibits significantoptical clarity and increased light transmission.

EXAMPLE 13

The procedure of Example 11 was repeated, using an 0.1 mm thick neutralgrey Llumar film (All Purpose Glass Coating Co., Clifton, N.J.). Thetransmission spectra for both the uncoated and the coated laminatedglass are shown in FIG. 4. The increase in transmission corresponds to areduction in the "mirror surface" of the window film. This is especiallyimportant for computer glare reduction screens.

EXAMPLE 14

A 3×5 cm piece of untreated polyester film (Hostaphan HC-4000, from 3MCorp.) was cleaned with methanol in an ultrasonic cleaner, the dipcoated in a solution of the polymer blend of Example 6 followd by dryingand baking, as above described. Transmission spectra for the coated anduncoated film are shown in FIG. 5. Significant enhancement in opticalclarity is obtained as a result of the coating.

Since various changes may be made in the invention without departingfrom its spirit and essential characteristics, it is intended that allmatter contained in the description shall be interpreted as illustrativeonly and not in a limiting sense, the scope of the invention beingdefined by the appended claims.

We claim:
 1. The method of making an anti-reflection coated body havinga reflective surface which comprises applying a solution of a polymericcomposition comprising a blend of(a) from about 1 to about 95 percent byweight of amorphous fluoropolymer, with (b) from about 5 to about 99percent by weight of a fluorinated copolymer having a polymer chaincomposed of

    --X.sub.s --Y.sub.t --Z.sub.u ]

units, wherein s, t and u represent weight proportions of the respectiveX, Y and Z units, and have values within the ranges ofs=from about 0.5to about 0.995; t=from about 0.0025 to about 0.4975; and u=from about0.0025 to about 0.4975; with the sum of s+t+u being 1; X representsunits of the composition ##STR8## wherein R¹ is H, --CH₃, or mixturesthereof;p is 1 or 2; n is an integer of from about 1 to about 40; Yrepresents units of the composition ##STR9## wherein R² is H, --CH₃ or--CH₂ COOH; Z represents units of the composition ##STR10## wherein R³is H, --CH₃, or CH₂ COOC_(m) H_(2m+1), wherein m is an integer of fromabout 1 to about 4, andR⁴ is an alkylene bridging group, straight chain,branched or cyclic, having from 1 to about 8 carbon atoms;wherein the X,Y and Z units may be arranged in any sequence.
 2. The method of claim 1wherein the amorphous fluoropolymer in said blend is a copolymer oftetrafluoroethylene and a comonomer selected from the group consistingof CH₂ ═CHF; CH₂ ═CF₂ ; CF₂ ═CHF; CH₂ ═CH═C_(N) F_(2N+1) ; CF₂ ═CC1F;CF₂ ═CF--CF₃ ; CF₂ ═CF--O--C_(N) F_(2N+1) ; CF₂ ═CF--O--CF₂CF(CF₃)--O--CF ₂ CF₂ SO₂ F; CF₂ ═CF--O--CF₂ CF(CF₃)--O--CF₂ CF₂ COOCH₃and ##STR11## and mixtures thereof.
 3. The method of claim 1, furthercomprising heating the coating at temperature between about 120° C. andabout 180° C. for time sufficient to effect cross-linking of saidfluorinated copolymer.
 4. The method of claim 2, further comprisingheating the coating at temperature between about 120° C. and about 180°C. for time sufficient to effect cross-linking of said fluorinatedcopolymer.
 5. The method of claim 1, wherein the solution of saidcopolymer is applied by spin-coating.
 6. The method of claim 1, whereinthe solution of said copolymer is applied by means of dipping said solidbody into said solution.